Phenomenon and Nature of Monolayer Dispersion

In the 1970s we began to be interested in the phase composition of many commercial catalysts that are based on supports with highly specific surfaces we found that oxides or salts, as active components in these catalysts, even though present in considerable quantity, did not show up in the X-ray diffraction (XRD) patterns. For example, Fig. 1 shows XRD patterns for some of these catalysts. The content of transition metal oxides or salts in these catalysts is of the order of 10% by weight, but they still fail to give peaks in XRD patterns. The patterns are merely those of amorphous supports. 

In general, it has been taken for granted that the disappearance of XRD peaks of an active component in a catalyst is due to the fact that the active component is present in such a small quantity as to evade detection by XRD. For example, in a supported noble metal catalyst, such as a platinum-reforming catalyst, Pt/y-Al203, the metal content, of the order of 0.1%, is too low to be detectable by XRD. However, this is not the case for the catalyst of our present interest. For the catalysts referred to in Fig. 

the content of the active component is about two orders of magnitude higher. Normally, for crystalline transition metal compounds, a content of the order of 1% by weight is sufficient to give rise to sharp peaks in the XRD pattern. Patterns a and f in Fig. 1 provide a convincing illustration of this point. 

A comparison of XRD patterns a and a or f and f in Fig. 1 raises the question as to what has become of the active component, the quantity of which is about 10% by weight in a catalyst. First of all, the active components no longer exist in the crystalline state. It is also unlikely that they have become an amorphous mass. Later on we shall provide further evidence to verify that the active component in each case has not formed stoichiometric compound with the support nor has dissolved in the support to form solid solution. It is then reasonable to think that the active component has dispersed as a monolayer on the surface of the support. In view of the high surface area of the supports and the heavy metals in the active components of these catalysts, a monolayer may still amount to a considerable percentage, usually more than 10% by weight, of an active 

According to a simple model based on the assumption that the anions of oxide or salt form a close-packed monolayer on the surface of the support and the cations occupy the interstices left over by anions, one can figure out the close-packed monolayer capacity for oxide or salt on a unit area of the support. We estimate it at 0.10 g/100 m2 or higher for various active components (see later, Table II). The specific surface of the support is about 200 m2/g for y-Al203, 300 m2/g for silica gel, and 1000 m2/g for active carbon. Although each of the catalysts in Fig. 1 contains a considerable amount of active component, its content is still lower than that estimated on the basis of a close-packed monolayer. Therefore, the monolayer dispersion in many of these catalysts does not correspond to the full coverage of the support surface, and more precisely is known as submonolayer dispersion. 

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